Laser radars have been adapted to transmit a continuous linear frequency modulated chirp pulse signal through the radar antenna aperture used for both transmission and reception for obvious cost and space savings. Higher and continuously improved resolution of three dimensional range image detection through pixelized laser radar is desired for better identification of targets. For example, there is a continuing desire to more accurately detect the size and shape of unidentified flying targets in the far field so as to better distinguish their differences, for example, the difference in size and shape of different aircraft.
Laser radars continuously transmitting linear frequency modulated chirp signals can be used to generate more precise pixelized three dimensional images for improved target recognition. In such laser radar systems, the transmitted and received signals propagate simultaneously through same antenna aperture. Retro-reflection of the transmitted signals at the transmission antenna generates unwanted noise signals commonly known as narcissus signals which can be 50 db higher than the desired reflected target signals. The narcissus signals, that is, the optical feedback, are generated by various physical phenomenon. The narcissus signals are generated by back-scattering caused by imperfection in and debris on the antenna surface, are generated by retro-reflection from the antenna, and are generated by diffraction from the transmitter, as is well known.
The simultaneous transmission and reception through the same antenna aperture in combination with the back-scattering, retro reflection and diffraction of the transmitted signals at the antenna aperture create large unwanted narcissus signals tending to obscure reflected target signals thereby creating a continuing need to improve the filtering of such unwanted narcissus signals. The unwanted narcissus signals are so large in comparison to the reflected target signals that they tend to swamp out or completely obscure the desired reflected target signal limiting improved target resolution, recognition and identification, as is well known. Thus, there exists a continuing need to improve narcissus filtering particularly useful in continuous laser radar systems.
Typically, the laser transmitter projects a diverging beam onto the antenna lens which then projects a collimated laser beam into the far field to reflect off of the target of interest. The lens further acts to focus the received reflected target signal from the far field upon a receiving diplexer directing the received target signal onto a detector. Thus, the laser transmit and target receive signals continuously and simultaneously traveling through the same antenna focal space between the antenna lens and the laser transmitter. The narcissus signals also continuously and simultaneously flood this same focal space. A diplexer, e.g., a polarized sensitive mirror, disposed between the laser transmitter and the antenna lens allows the transmit signal to propagate to the lens, while reflecting the received target signal onto an optical detector. However, the diplexer not only directs the target signal onto the detector, but also directs a significant portion of the narcissus signal onto the optical detector, with the unwanted narcissus signals tending to swamp out and obscure the desired target signal.
Detectors will generate noise due to their quantum mechanics and have an inherent dynamic range between the maximum signal processed and the noise level generated. It is desired that the narcissus signal be filtered to below the noise level of the detector so that the narcissus signal is reduced as much as possible to improve the detection of the target signal. Taking advantage of the entire dynamic range of the detector achieves maximum narcissus filtering possible to improve the ability to detect the target signal. Thus, target signal detection is detector-noise limited and the maximum narcissus filtering possible is limited to the dynamic range of the input detectors. Optical detectors have a higher dynamic range than RF detectors, and the optical detectors enable superior narcissus filtering over the RF detectors. Hence, there exists a continuing need to provide narcissus rejection filtering with better than 50 db in attenuation using detectors with the highest dynamic range in excess of 50 db to allow sufficient and maximum possible narcissus filtering to detect as best possible the target signal.
Various types of electronic narcissus filtering have been used including cancellation and fixed frequency narcissus notch filtering. Cancellation is performed by generating a signal which is equal in amplitude, but opposite in phase to the narcissus signal such that by summing the two, the narcissus signal is canceled and reduced. However, the cancellation method has been shown to provide up to 40 db maximum attenuation and is insufficient to remove the 50 db relative signal difference between the target signal and the narcissus signal. Moreover, for cancellation to function properly, the amplitude and the frequency of the narcissus signal must be known at all times. The amplitude and dynamic frequency of the narcissus is difficult to predict or accurately determine at all times with sufficient precision for cancellation to adequately perform.
Fixed frequency notch filters used in non-modulated laser radars use a reference signal at the same frequency of the narcissus signal to shift the target return signal and the narcissus signals within the bandwidth of a notch band rejection filter. These notch filters have a filtering band for attenuating a specific unwanted frequency signal, i.e., a fixed frequency narcissus signal. However, the bandwidth of the fixed frequency notch rejection filters have been too broad to allow for isolation of the frequency shifted target signals having frequencies in close proximity to the fixed frequency notch filter band.
Fixed frequency notch filtering has failed to successfully eliminate narcissus signals in a continuous linear modulated frequency laser radar system. Moreover, such fixed frequency notch narcissus filters do not provide for dynamic filtering of the modulated narcissus signals. In continuous radar systems using continuous linear frequency modulated chirp signals, the modulating signals generate frequency modulated narcissus signals having a dynamic frequency range through the modulated transmitted frequency range, rendering fixed frequency notch filters useless in frequency modulated laser systems. Hence, there exists a need for modulated narcissus signal filtering with up to 60 db attenuation. The disadvantages and limitations of the prior art fixed frequency notch filters and cancellation methods are solved using the present invention.